This invention relates in general to photolithography and, more particularly, to a photomask and a method for forming an opaque border on the same.
Phase shift masks improve wafer imaging by using both the intensity and phase of electromagnetic radiation (EMR) in a photolithography system to improve the image contrast on a wafer. Phase shift masks can generally produce smaller geometries on wafers than traditional binary masks. While a binary mask generally only modulates the amplitude of the EMR, a phase shift mask typically modulates both the amplitude and the phase of the EMR in a way that may improve resolution of a projected image.
One particular type of phase shift mask, known as an embedded phase shift mask, typically includes a patterned layer of partially transmissive material disposed on a transparent substrate. The patterned layer is generally located in an area to be imaged onto a wafer (e.g., in a mask field). However, rather than being opaque to an exposure wavelength, the patterned layer generally transmits a small percentage (e.g., approximately one to thirty percent) of the exposure EMR. A common material used in embedded phase shift masks to form the partially transmissive layer is molybdenum silicide (MoSi). EMR that passes through the partially transmissive layer may be reduced in intensity and is typically one hundred and eighty degrees (180xc2x0) out of phase with respect to the EMR transmitted only through the transparent substrate. At the feature edges, the EMR passing through the transparent substrate may merge with the EMR passing through the partially transmissive layer to produce zero intensity as a result of the destructive interference. As a result, the image produced on the wafer often has sharper edges and a better resolution than an image produced by a binary mask.
An embedded phase shift mask may be a weak phase shift mask, in which the amount of the phase shift is proportional to the percentage of EMR transmitted through the partially transmissive layer. If the partially transmissive layer has a low transmission, the increase in resolution of the image on the wafer may be small. If the partially transmissive layer has a high transmission, the increase in resolution may be greater, but any additional exposure due to background illumination may become strong enough to create an exposed area that was designed to be unexposed. Consequently, in a photolithography system using a weak phase shift mask, EMR transmission by the mask should typically be strictly limited to the mask field.
For a binary mask, it may be possible to reduce transmission of background EMF by leaving the chrome layer that was used to define the pattern on the border regions. In embedded phase shift masks, however, the MoSi layer may transmit more EMR than is allowable outside of the mask field. For instance, EMR through the MoSi may double the background exposure around the edges of the mask field and quadruple background exposure at the corners of the mask field.
A field aperture and possibly other elements, such as stepper blades, may be used in an attempt to prevent EMR transmission from any border regions (i.e., regions outside the mask field). For instance, stepper blades may prevent EMR from reaching the wafer via peripheral structures located in border regions. However, the aperture, stepper blades, and other elements may not be exact, and the lithography system may still suffer from excess background illumination.
One solution to this problem involves using two patterning operations to create an embedded phase shift mask with two patterned layers. Such a process is described in U.S. Pat. No. 5,741,613 (hereinafter, the xe2x80x9c613 patentxe2x80x9d ). The process in the 613 patent results in two patterned layers: one patterned layer formed from partially transmissive material and the other patterned layer formed from opaque material.
Specifically, in the process described by the 613 patent, a photomask blank includes a transparent substrate, a layer of partially transmissive material on the substrate, a layer of opaque material on the partially transmissive material, and a layer of resist on the opaque material. In a first patterning operation, a pattern is formed in the opaque and partially transmissive materials by selectively exposing then developing the resist, and then etching the opaque and partially transmissive layers in the regions no longer covered by the resist. The first resist layer is then removed. Then, in a second patterning operation, a second resist layer is formed on the mask, covering the patterned opaque and partially transmissive layers. The second layer of resist is exposed in the shape of a large window the size of the desired mask field. Also, the second patterning operation includes the steps of developing the resist and removing the opaque material from the areas no longer covered by the resist. Any remaining resist is then removed, to leave an opaque layer covering the border region of the mask surrounding the mask field. The opaque layer may absorb exposure EMR and thereby prevent any EMR that strikes the border region from being exposed onto the wafer.
However, masks are costly to manufacture in the manner described by the 613 patent, since two patterning operations are required. Also, when initially preparing the photomask blank, mask defects may be created during deposition of the partially transmissive layer on the transparent substrate, and during deposition of the opaque layer on the partially transmissive layer. Such masks will therefore typically have approximately twice the number of blank defects than for a binary mask that only requires one layer.
In accordance with the teachings of the present invention, disadvantages and problems associated with producing a photomask with an opaque border around a mask field have been substantially reduced or eliminated.
In an example method of manufacturing a photomask, no more than one patterning operation is used to form a mask field with a pattern of features and a border region substantially covered by an opaque material substantially surrounding the mask field. The mask field may include partially transmissive material. When the photomask is used to expose a pattern on an object, the opaque material may substantially prevent electromagnetic radiation (EMR) from passing through the border region.
In accordance with one embodiment of the present invention, when the mask field is formed, an insulating clear region may be formed in the mask field surrounding the pattern, and the border may be disposed outside the insulating clear region. The opaque layer may then be deposited onto the border region and adhered to the border region by electrodeposition.
In accordance with another embodiment of the present invention, the opaque material may be adhered to the border by selectively spraying the opaque material into the desired coverage areas, for example in a process like ink-jet printing. For embodiments in which the opaque material is printed onto the border region, the insulating clear region may be omitted.
A technical advantage of certain embodiments of the present invention is that the insulating clear region substantially prevents the opaque material from being deposited on or adhering to features in the pattern.
Another technical advantage of certain embodiments of the present invention is that the photomask may be manufactured with a single patterning operation. Other technical advantage of certain embodiments may include reduced manufacturing time, reduced manufacturing expense, reduced mask defects.
All, some, or none of these technical advantages may be present in various embodiments of the present invention. Other technical advantages will be readily apparent to one skilled in the art from the following figures, descriptions, and claims.